The production of sustainable transportation fuels and commodity chemicals from lignocellulosic biomass is a major component of the international renewable energy technology portfolio, which will ultimately provide significant energy, economic, and climate security for the world. However, plant cell walls are highly evolved heterogeneous composite structures, which represent a significant challenge to deconstruct selectively. The majority of the sugars locked in plant cell walls are from cellulose and hemicellulose, with the former being more recalcitrant to deconstruction.
To date, many processes have been developed to produce fuels from biomass-derived sugars, ranging from ethanol via fermentation to higher alcohols from genetically-modified organisms to hydrocarbons produced biologically or catalytically. Thus, over a wide range of fuel production options, there is significant impetus to develop cost-effective methods to produce sugars for upgrading to fuels and commodity chemicals. The current leading industrial option to produce sugars from lignocellulosic biomass utilizes a thermochemical pretreatment step that renders the plant cell wall more amenable to the effective application of enzyme cocktails that deconstruct cellulose and hemicellulose to soluble sugars. The enzymatic hydrolysis step alone represents a significant fraction of the operating and capital cost of lignocellulosic biofuel production. Most enzyme cocktails under development today are based on fungal or bacterial cellulase secretomes. The industrial emphasis on fungal cocktails originated from the United States Army's isolation of the fungus Trichoderma reesei (anamorph of Hypocrea jecorina) in the South Pacific in the late 1940s, which has grown into an important platform for the production of cellulases at extremely high protein titers. The use of bacterial cellulase cocktails has focused effort on both free cellulase systems and complexed enzyme (i.e., cellulosomal) systems, as well as engineering of cellulase-producing bacteria and fungi to produce fuels and chemicals directly in a process known as Consolidated Bioprocessing. In the fungal enzyme cocktails, the processive cellulases are the primary components, and provide the majority of the hydrolytic activity for cellulose conversion to glucose. The processive cellulases have thus been the focus of many structural and biochemical studies and the primary targets for cellulase engineering.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.